Method for production of valve head portion of hollow engine valve and hollow engine valve

ABSTRACT

Forming of a valve head portion ( 1 ) of a hollow engine valve (V) comprising a material particularly satisfactory in heat resistance is performed by cold forging. On this occasion, the material with excellent heat resistance is worked to form a valve head portion semifinished product ( 11 ) having a hollow hole (S 11 ) whose inner diameter (Φ 11 ) is equal to the maximum inner diameter (Φ 11 ) of a hollow hole (S 1 ) of a finished product of the valve head portion ( 1 ), and having a diameter-increased section ( 111 ) whose maximum outer diameter (Φ 12 ) is equal to the maximum outer diameter (Φ 12 ) of a diameter-increased section ( 1   a ) of the finished product. Parts of the valve head portion semifinished product other than a central part to a lower part of the diameter-increased section ( 111 ) are necked down by cold forging performed a plurality of times to obtain the finished product of the valve head portion ( 1 ). This method of obtaining the finished product is provided by the present invention.

TECHNICAL FIELD

This invention relates to a method for production of a valve headportion of a hollow engine valve; and the hollow engine valve having thevalve head portion. The valve head portion comprises a material havingheat resistance comparable to or better than the heat resistance of amaterial for a hollow shaft portion, and the valve head portion has avalve head portion hollow hole opening on a side welded to the hollowshaft portion. The valve head portion hollow hole is formed to have anincreased diameter in a diameter-increased section of the valve headportion, and the maximum inner diameter of the valve head portion hollowhole is larger than the maximum outer diameter of the hollow shaftportion.

BACKGROUND ART

For the production of a hollow engine valve, three members thereof,i.e., a valve head portion, a hollow shaft portion, and a shaft endsealing material, are prepared separately, and these three members arefinally joined together by welding to obtain a hollow engine valve as afinished product. This technology has so far been performed, asdisclosed in Patent Document 1 to be described below. It has also beenperformed to join a valve head portion and a hollow shaft portion, whichhas one end sealed, by welding, thereby obtaining a hollow engine valveas a finished product. Sodium is sealed up in or enclosed in a hollowhole of a hollow engine valve for an exhaust valve required to showhigh-temperature resistance, and takes charge of cooling in regions ofthe engine valve, where the temperature is particularly high, rangingfrom the valve head portion to a section of the hollow shaft portioncloser to the valve head portion. As the shape of the hollow hole of thehollow engine valve, therefore, a shape having a diameter increasedinside the valve head portion, as shown in FIG. 1 of Patent Document 2to be described later, is more desirable than a simple cylindrical shapeas seen in FIGS. 2 and 3 of the Patent Document 1 described later.

With the conventional hollow engine valve, especially, the hollow enginevalve for exhaust which is exposed to high temperatures, it has beencommon practice to use a material showing excellent characteristicsinvolving heat resistance, such as a manganese-based heat resistingsteel, or a nickel-based heat resisting steel, in the valve head portionwhere the temperature is highest; use an ordinary steel material or anordinary heat resisting steel material for the hollow shaft portion orthe shaft end sealing material where the temperature is not so high; andweld the three members, i.e., the valve head portion, hollow shaftportion, and shaft end sealing material, or the two members, i.e., thevalve head portion and the hollow shaft portion sealed at one end,thereby constructing a finished product of a hollow engine valve.Alternatively, if particularly excellent heat resistance is required, amethod of using a material showing excellent characteristics involvingheat resistance for the hollow shaft portion, as for the valve headportion, has been performed.

Essentially, it is ideal to produce the entire hollow engine valve fromany of the above-mentioned materials showing characteristics includingexcellent heat resistance. However, these materials are expensive, hard,and poor in work characteristics or workability. Thus, these materialsare used only for the valve head portion where the temperature isparticularly high, and an ordinary steel plate is applied to the hollowshaft portion or the shaft end sealing material where the temperature isnot very high. With the methods for producing hollow engine valves,therefore, numerous technological developments have been made,particularly, in regard to the method for production of the valve headportion. There are not many cases of using the material showingcharacteristics including excellent heat resistance for the hollow shaftportion as well. Thus, emphasis will hereinafter be placed ontechnologies concerned with methods for producing the valve head portionwhich, in the exhaust valve, needs to use the material showingcharacteristics including excellent heat resistance and whose forming isdifficult.

In the production of the valve head portion of the hollow engine valve,the point of development concentrates on two areas. The first area is amethod of producing the leading end diameter-increased section of thevalve head portion, and the second area is a technology on a method ofproviding a hollow hole in the valve head portion. That is, the materialused for the valve head portion is excellent in heat resistance, butpoor in workability, as mentioned above. Thus, when the leading enddiameter-increased section is to be formed, or when the hollow hole isto be provided, a high degree of difficulty in working has to beovercome in comparison with the working of an ordinary steel material.In this connection, various inventions and devices have beenaccomplished thus far.

In the Patent Document 1, “a hollow valve formed by rolling up a steelplate” does not touch on details of the method for production of thevalve head portion, but merely describes “forging”. Since the hollowhole therein is in a cylindrical shape, it is assumed to have beenformed using a punch or a drill.

In “a method for production of a Na-filled hollow engine valve”disclosed in the Patent Document 2, “hot plastic working” is describedas being used to form the diameter-increased section of the valve headportion, and “hot isostatic pressing” is described as being used to formthe hollow hole. It follows, therefore, that “hot forging” forms both ofthem. In the invention of the Patent Document 2, “a round material of alarger diameter than the diameter of the valve shaft” is described asbeing used as the material for the valve head portion, and the innerdiameter of the hollow hole is described as being initially “larger thanthe inner diameter of the finished valve hollow portion”. In FIG. 1 ofthe Patent Document 2, the initial inner diameter of the hollow hole isfurther increased during the hot forging process of the valve headportion, and the hollow hole increased in diameter is depicted as beingprovided inside the diameter-increased section of the valve headportion. In the actual forging process, however, it is very difficult toincrease the diameter of the hollow hole to a dimension equal to orlarger than the inner diameter of the hollow hole initially bored,thereby forming the hollow hole into an appropriate shape. Actually, thehollow hole is not increased in diameter, unlike that depicted in FIG. 1of the Patent Document 2.

In “a method for production of a Na-filled hollow engine valve”disclosed in Patent Document 3 to be described later, the leading end ofa pipe is first forged into a spherical form, and then formed into anumbrella shape, according to the first invention and the secondinvention of this document. The leading end diameter-increased sectionof the valve head portion is formed by this method, and this method offorming is hot forging. The difference exists that the hollow piperemains as such in the first invention, while a highly machinablematerial is sealed up in the hollow portion in the second invention. Thethird invention, on the other hand, starts with a pipe having a highlymachinable material enclosed in its hollow portion, the pipe having alarger diameter than the diameter of a hollow shaft portion to beformed, and forms one end of the pipe into an umbrella shape, and theother end of the pipe into the shape of a thin pipe. The method offorming is hot forging for both ends.

As far as the formation of the hollow hole is concerned, the firstinvention has the hollow hole from the start, whereas the secondinvention and the third invention form the hollow hole by cutting andremoving the highly machinable material enclosed in the hollow portion.In all of the first to third inventions, the center of the leading enddiameter-increased section of the valve head portion is lacking in thematerial with high resistance to heat. These inventions, however, adopta method in which the highly heat resistant material is welded to thiscentral part in the final process, whereby the entire outside of theleading end diameter-increased section is formed as comprising thehighly heat resistant material.

In “a method for producing a Na-filled hollow engine valve” disclosed inPatent Document 4 to be described below, the valve head portion is of aconglomerate form (first invention) or in the form of a round bar(second invention), and is characterized by having a shallow hole in anend part thereof joined to the hollow shaft portion. That is, the highlyheat resistant material constituting the valve head portion has poormachinability. Thus, only the shallow hole is bored, and the hollowshaft portion is welded to the bored part to form a continuous hollowhole, whereafter the valve head portion is formed into an umbrellashape. The steps of forming the valve head portion are performed by hotforging. Drawbacks characteristic of this method are as follows: Thehollow hole bored in the valve head portion is the shallow hole. In thehollow engine valve as the finished product, therefore, the site ofwelding between the valve head portion and the hollow shaft portion isin the close vicinity of the diameter-increased section of the valvehead portion, and is exposed to considerably high temperatures duringthe operation of the engine. This arouses concern about strength.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-63-195308-   Patent Document 2: JP-A-7-102917-   Patent Document 3: JP-A-7-119421-   Patent Document 4: JP-A-7-208127

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As far as the above-mentioned conventional technologies are concerned,hot forging is used for the formation of the valve head portion. For theformation of the hollow hole, the first step is to enclose the highlymachinable material within the pipe of the highly heat resistantmaterial, and the finishing step is to cut the highly machinablematerial by a gun drill or the like, thereby providing the hollow hole.Alternatively, the highly heat resistant material is used in the form ofa pipe from the beginning to utilize its hollow hole unchanged.Furthermore, the hollow hole of the valve head portion is formed as ashallow hole, and a pipe composed of ordinary steel is connected to thevalve head portion to complete the hollow hole. As seen here, variouscontrivances have been tried in order to overcome the minimalworkability of the highly heat resistant material of the valve headportion.

In connection with the use of hot forging for the formation of the valvehead portion, however, the following problems have been pointed out: Inthe case of hot forging, the temperature of the material has to beraised to a value of the order of 850° C. to 1,200° C., thus requiringfacilities for this purpose. Also, time is taken for cooling, so thatlabor is increased for working. Moreover, hot forging, which involvesthe problem of metal expansion or the like, cannot be expected toachieve working accuracy as high as that of cold forging. Furthermore,the texture of the surface of the resulting product is poor comparedwith that in cold forging.

Hence, it is ideal that the forming of the valve head portion by workingshould be performed, in essence, by cold forging, or warm forging, atmost. For the valve head portion of the hollow engine valve as anexhaust valve, in particular, use has to be made of manganese-based ornickel-based steel materials with excellent heat resistance. However,such materials are so hard that they are markedly inferior inworkability, as stated above, and it has been considered impossible toform them into the finished shape of the valve head portion by coldforging.

These facts are also described explicitly in the Patent Document 3(paragraph 0006) and the Patent Document 4 (paragraph 0008). Therelevant descriptions in the Patent Document 4 will be cited below.

“With SUH35 (21% Cr-4% Ni-9% Mn steel), which becomes mainstream forexhaust valves, Mn is contained in a large amount during cold working atthe time of finishing. Thus, work hardening occurs, making pipeformation extremely difficult. Current technologies for pipe formationcause fine cracks or ruptures during cold formation of pipes having anouter diameter of 10 mm or less, thereby presenting a bottleneck intheir practical use.”

The expression of the components of SUH35 in the above citation is thesame as the expression in the Patent Document 4.

The formation of the hollow hole also poses various problems. The hollowhole in the valve head portion should desirably be not of a simplecylindrical form of the same cross-section, but be increased in diameterin the interior of the leading end diameter-increased section, where thetemperature is the highest, in correspondence with thediameter-increased shape of the external surface. This is the case,needless to say, from the point of view of the cooling action of sodiumenclosed in the hollow hole. In the state where the shallow hole isbored, however, it is impossible to increase the diameter of the hollowhole within the leading end diameter-increased section, as observed inthe invention of the Patent Document 4. If, instead, the material forthe valve head portion is rendered of a pipe shape from the beginning,as in the invention of the Patent Document 3, the final step has to beto weld a highly heat resistant material separately to the leading endof the pipe. This is laborious, and concern about strength due towelding remains unsolved, because the leading end diameter-increasedsection of the valve head portion is a site exposed to particularly hightemperatures. Besides, in the invention of the Patent Document 2, thehollow hole of the leading end diameter-increased section of the valvehead portion does not become the one illustrated in FIG. 1 of the PatentDocument 2, as discussed earlier.

Based on the foregoing, the problems with or targets for the presentinvention have been set as follows:

<Target 1>

To develop a method for forming the valve head portion, which comprisesa material having heat resistance comparable to or better than that ofthe material for the hollow shaft portion in a hollow engine valve, bycold forging.

<Target 2>

To obtain a hollow hole increased in diameter in the leading enddiameter-increased section of the valve head portion comprising theabove material such that the maximum inner diameter of the hollow holeis larger than at least the outer diameter of a rear end part of thevalve head portion, namely, a junction with the hollow shaft portion.

Means for Solving the Problems

The present invention has been accomplished to solve the problems asmeans for reaching the targets, and provides the following solutions:

<Solution 1>

A method for production of a valve head portion of a hollow enginevalve, in which the hollow engine valve has a hollow shaft portion, andthe valve head portion comprising a material having heat resistancecomparable to or better than heat resistance of a material for thehollow shaft portion, the valve head portion has a valve head portionhollow hole opening on a side welded to the hollow shaft portion, thevalve head portion hollow hole is formed to have an increased diameterin a diameter-increased section of the valve head portion, and a maximuminner diameter of the valve head portion hollow hole is larger than amaximum outer diameter of the hollow shaft portion,

the method comprising:

a first step of producing a valve head portion semifinished product byworking the material having the heat resistance comparable to or betterthan the heat resistance of the material for the hollow shaft portion toform the valve head portion semifinished product,

in which the valve head portion semifinished product has a cylindricalbarrel section and a diameter-increased section at one end of the barrelsection, the diameter-increased section being integral with the barrelsection,

when the diameter-increased section is located below, a maximum outerdiameter of the diameter-increased section is equal to a maximum outerdiameter of the diameter-increased section of the valve head portion asa finished product, and

the valve head portion semifinished product has a cylindrical hollowhole having an inner diameter equal to the maximum inner diameter of thevalve head portion hollow hole of the finished product, the cylindricalhollow hole opening at an upper end, and having a lower end bottomed inthe diameter-increased section,

the first step including:

using a cylindrical solid round bar as a material, the solid round barcomprising the material having the heat resistance comparable to orbetter than the heat resistance of the material for the hollow shaftportion, the solid round bar having a diameter larger than a minimumouter diameter of the valve head portion as the finished product, butsmaller than a maximum outer diameter of the valve head portion as thefinished product;

while rendering a circular surface at one end of the solid round bar anupper surface, and a circular surface at another end of the solid roundbar a lower surface, forming a cylindrical hollow hole opening at anupper end, and being bottomed at a lower end, an inner diameter of thecylindrical hollow hole being equal to the maximum inner diameter of thevalve head portion hollow hole of the valve head portion as the finishedproduct; and

then increasing a diameter of a lower part of the solid round bar byforging to bring a maximum outer diameter of the lower part intoconformity with the maximum outer diameter of the diameter-increasedsection of the valve head portion as the finished product,

thereby producing the valve head portion semifinished product having thediameter-increased section whose maximum outer diameter coincides withthe maximum outer diameter of the diameter-increased section of thevalve head portion as the finished product, the valve head portionsemifinished product having the cylindrical hollow hole having the innerdiameter equal to the maximum inner diameter of the valve head portionhollow hole of the valve head portion as the finished product, thecylindrical hollow hole having the lower end bottomed; and

a second step of gradually necking down an upper part of thediameter-increased section and the barrel section of the valve headportion semifinished product by cold forging in a plurality of stages,that is, gradually necking down the upper part of the diameter-increasedsection and the barrel section by use of dies for pressing the upperpart of the diameter-increased section and the barrel section, innerdiameters of the dies being decreased little by little as the stageadvances, a number of the dies used being equal to a number of processesfor necking,

thereby obtaining the valve head portion as the finished productconfigured such that the maximum inner diameter of the valve headportion hollow hole in the diameter-increased section is maintained atthe inner diameter of the cylindrical hollow hole, the inner diameter ofthe valve head portion hollow hole being decreased with increasingheight and being equal to an inner diameter of a hollow hole of thehollow shaft portion at an upper end of the barrel section.

<Solution 2>

A method for production of a valve head portion of a hollow enginevalve, in which the hollow engine valve has a hollow shaft portion, andthe valve head portion comprising a material having heat resistancecomparable to or better than heat resistance of a material for thehollow shaft portion, the valve head portion has a valve head portionhollow hole opening on a side welded to the hollow shaft portion, thevalve head portion hollow hole is formed to have an increased diameterin a diameter-increased section of the valve head portion, and a maximuminner diameter of the valve head portion hollow hole is larger than amaximum outer diameter of the hollow shaft portion,

the method comprising:

a first step of producing a valve head portion semifinished product byworking the material having the heat resistance comparable to or betterthan the heat resistance of the material for the hollow shaft portion toform the valve head portion semifinished product,

in which the valve head portion semifinished product has a cylindricalbarrel section and a diameter-increased section at one end of the barrelsection, the diameter-increased section being integral with the barrelsection,

when the diameter-increased section is located below, a maximum outerdiameter of the diameter-increased section is equal to a maximum outerdiameter of the diameter-increased section of the valve head portion asa finished product, and

the valve head portion semifinished product has a cylindrical hollowhole having an inner diameter equal to the maximum inner diameter of thevalve head portion hollow hole of the finished product, the cylindricalhollow hole opening at an upper end, and having a lower end bottomed inthe diameter-increased section,

the first step including:

using a cylindrical solid round bar as a material, the solid round barcomprising the material having the heat resistance comparable to orbetter than the heat resistance of the material for the hollow shaftportion, the solid round bar having a diameter larger than a minimumouter diameter of the valve head portion as the finished product, butsmaller than a maximum outer diameter of the valve head portion as thefinished product;

while rendering a circular surface at one end of the solid round bar anupper surface, and a circular surface at another end of the solid roundbar a lower surface, increasing a diameter of a lower part of the solidround bar by forging to bring a maximum outer diameter of the lower partinto conformity with the maximum outer diameter of thediameter-increased section of the valve head portion as the finishedproduct,

then forming a cylindrical hollow hole opening at an upper end, andbeing bottomed at a lower end, an inner diameter of the cylindricalhollow hole being equal to the maximum inner diameter of the valve headportion hollow hole of the valve head portion as the finished product;and

thereby producing the valve head portion semifinished product having thediameter-increased section whose maximum outer diameter coincides withthe maximum outer diameter of the diameter-increased section of thevalve head portion as the finished product, the valve head portionsemifinished product having the cylindrical hollow hole having the innerdiameter equal to the maximum inner diameter of the valve head portionhollow hole of the valve head portion as the finished product, thecylindrical hollow hole having the lower end bottomed; and

a second step of gradually necking down an upper part of thediameter-increased section and the barrel section of the valve headportion semifinished product by cold forging in a plurality of stages,that is, gradually necking down the upper part of the diameter-increasedsection and the barrel section by use of dies for pressing the upperpart of the diameter-increased section and the barrel section, innerdiameters of the dies being decreased little by little as the stageadvances, a number of the dies used being equal to a number of processesfor necking,

thereby obtaining the valve head portion as the finished productconfigured such that the maximum inner diameter of the valve headportion hollow hole in the diameter-increased section is maintained atthe inner diameter of the cylindrical hollow hole, the inner diameter ofthe valve head portion hollow hole being decreased with increasingheight and being equal to an inner diameter of a hollow hole of thehollow shaft portion at an upper end of the barrel section.

<Solution 3>

The method for production of a valve head portion of a hollow enginevalve according to Solution 1 or Solution 2, further comprising, in thesecond step:

installing a ram where the dies are fixed, and a press bed where worksare fixed, at spaced locations with the ram being located above and thepress bed being located below;

fixing N of the dies, which corresponds to a number of the processes (Nof the processes) for cold forging, at equally spaced intervals to theram in a sequence of the processes for necking, N being a positiveinteger and N ≧2;

locating the valve head portion semifinished products on the press bedsuch that a first of the valve head portion semifinished products liesbelow a first of the dies;

lowering the ram to perform first forging of the first valve headportion semifinished product for necking by the first die;

raising the ram, and transfer-moving the first valve head portionsemifinished product to be located below a second of the dies, andsimultaneously locating a second of the valve head portion semifinishedproducts below the first die;

lowering the ram to perform second forging of the first valve headportion semifinished product for necking by the second die, andsimultaneously perform first forging of the second valve head portionsemifinished product for necking by the first die;

raising the ram, and transfer-moving the first valve head portionsemifinished product to be located below a third of the dies, and thesecond valve head portion semifinished product to be located below thesecond die, and simultaneously locating a third of the valve headportion semifinished products below the first die;

lowering the ram N times and transfer-moving a plurality of the valvehead portion semifinished products, in such a manner, and when the firstvalve head portion semifinished product becomes a valve head portionfinished product as a first valve head portion finished product,removing the first valve head portion finished product from the pressbed; and

further continuing the processes to obtain a plurality of the valve headportion finished products.

<Solution 4>

The method for production of a valve head portion of a hollow enginevalve according to Solution 1 or Solution 2, further comprising, in thesecond step:

installing a ram where the dies are fixed, and a press bed where worksare fixed, at spaced locations with the ram being located below and thepress bed being located above;

fixing N of the dies, which corresponds to a number of the processes (Nof the processes) for cold forging, at equally spaced intervals to theram in a sequence of the processes for necking, N being a positiveinteger and N ≧2;

locating the valve head portion semifinished products on the press bedsuch that a first of the valve head portion semifinished products liesabove a first of the dies;

raising the ram to perform first forging of the first valve head portionsemifinished product for necking by the first die;

lowering the ram, and transfer-moving the first valve head portionsemifinished product to be located above a second of the dies, andsimultaneously locating a second of the valve head portion semifinishedproducts above the first die;

raising the ram to perform second forging of the first valve headportion semifinished product for necking by the second die, andsimultaneously perform first forging of the second valve head portionsemifinished product for necking by the first die;

lowering the ram, and transfer-moving the first valve head portionsemifinished product to be located above a third of the dies, and thesecond valve head portion semifinished product to be located above thesecond die, and simultaneously locating a third of the valve headportion semifinished products above the first die;

raising the ram N times and transfer-moving a plurality of the valvehead portion semifinished products, in such a manner, and when the firstvalve head portion semifinished product becomes a valve head portionfinished product as a first valve head portion finished product,removing the first valve head portion finished product from the pressbed; and

further continuing the processes to obtain a plurality of the valve headportion finished products.

<Solution 5>

A hollow engine valve, comprising:

a hollow shaft portion opening at both ends;

a valve head portion produced by the method for production according toSolution 1 or Solution 2 or Solution 3 or Solution 4 and welded to oneof the ends of the hollow shaft portion; and

a shaft end sealing material welded to the other end of the hollow shaftportion.

<Solution 6>

A hollow engine valve, comprising:

a hollow shaft portion sealed at one end; and

a valve head portion produced by the method for production according toSolution 1 or Solution 2 or Solution 3 or Solution 4 and welded toanother end of the hollow shaft portion.

Effects of the Invention

According to the invention as Solution 1 or Solution 2, the method ofthe present invention has the first step of producing a valve headportion semifinished product by working a material having heatresistance comparable to or better than the heat resistance of thematerial for a hollow shaft portion to form the valve head portionsemifinished product, in which the valve head portion semifinishedproduct has a cylindrical barrel section and a diameter-increasedsection at one end of the barrel section, the diameter-increased sectionbeing integral with the barrel section, when the diameter-increasedsection is located below, the maximum outer diameter of thediameter-increased section is equal to the maximum outer diameter of thediameter-increased section of the valve head portion as a finishedproduct, and the valve head portion semifinished product has acylindrical hollow hole having an inner diameter equal to the maximuminner diameter of the valve head portion hollow hole of the finishedproduct, the cylindrical hollow hole opening at an upper end, and havinga lower end bottomed in the diameter-increased section. Thus, themaximum inner diameter of the valve head portion hollow hole provided inthe valve head portion as the finished product can be rendered largerthan the outer diameter of the hollow shaft portion. Hence, sodiumenclosed in the hollow hole can be filled within the hollow holeincreased in diameter in the diameter-increased section of the valvehead portion exposed to the highest temperature. As a result, cooling ofthe diameter-increased section of the valve head portion can beperformed efficiently. Furthermore, sealing of the diameter-increasedsection of the valve head portion with a highly heat resistant materialby welding is not performed. Instead, the highly heat resistant materialpacking the hollow hole of the diameter-increased section of the valvehead portion is all subjected to forming integrally. Thus, no labor isrequired for welding, and no concern about strength is caused.

That is, the semifinished product of the valve head portion is firstmanufactured which is provided with the hollow hole of the same innerdiameter as the maximum inner diameter of the valve head portion hollowhole provided in the valve head portion as the finished product.Afterwards, in the process for necking, with the bottomed end of thevalve head portion hollow hole being intact, other parts thereof arenarrowed, whereby a hollow hole of a desired inner diameter can beprovided in the diameter-increased section of the valve head portion.

Moreover, the maximum outer diameter of the diameter-increased sectionof the semifinished product of the valve head portion has beforehandbeen brought into agreement with the outer diameter of the largestdiameter-increased section of the valve head portion as the finishedproduct. Thus, even in the subsequent narrowing (necking) process(second step), no force needs to be exerted on this site, that is, thissite need not be pressed by the die. Consequently, no buckling orcracking occurs in the second step. That is, in order that neither theexterior nor the interior of the diameter-increased section,particularly, its middle to lower part, needs to be mostly worked in thesecond step, a shift toward the second step takes place after productionof the semifinished product in which the maximum outer diameter of thediameter-increased section and the maximum inner diameter of the hollowhole within the diameter-increased section are equalized with themaximum outer diameter of the diameter-increased section of the valvehead portion as the finished product and the maximum inner diameter ofthe hollow hole within the diameter-increased section of the valve headportion as the finished product, respectively. The cold forging processof the second step, therefore, is reduced to a very simple step in whichthe other parts, namely, the upper part of the diameter-increasedsection and the barrel section, are gradually necked down. This makespossible the cold forging of a particularly highly heat resistant steelmaterial which has so far been considered difficult.

According to the conventional concept, “cold forging of the valve headportion” has been considered to be one for forming the whole of thevalve head portion at a stroke by cold forging. On this occasion,difficulty in working the material has become a bottleneck, making suchcold forging unachievable, as stated previously. However, the inventorof the present invention anticipated as follows: It would suffice toform, initially, the maximum outer diameter part of thediameter-increased section of the valve head portion and the maximuminner diameter part of the hollow hole inside it into the same shapes asthose in the finished product. A subsequent step would be only to narrow(neck down) other parts a plurality of times. This method would enablecold forging to perform forming of the valve head portion. With thisanticipation, the inventor conducted experiments. That is, the inventorthought of dividing the entire forming process for the valve headportion into the first step of forming the semifinished product in whichthe maximum outer diameter of the diameter-increased section and themaximum inner diameter of the hollow hole inside it are in the samestate as the state of the finished product; and the second step ofnecking down other parts a plurality of times to obtain the finishedproduct. Experiments on this dividing method led to a success inobtaining a complete valve head portion finished product by coldforging, without buckling or cracking.

According to the invention as Solution 1 or Solution 2, the method ofthe present invention similarly has the second step of gradually neckingdown an upper part of the diameter-increased section and a barrelsection of the valve head portion semifinished product by cold forgingin a plurality of stages, that is, gradually necking down the upper partof the diameter-increased section and the barrel section of the valvehead portion semifinished product by use of dies for pressing the upperpart of the diameter-increased section and the barrel section, the innerdiameters of the dies being decreased little by little as the stageadvances, the number of the dies used being equal to the number ofnecking processes, thereby obtaining the valve head portion as thefinished product configured such that the maximum inner diameter of thevalve head portion hollow hole in the diameter-increased section ismaintained at the inner diameter of the cylindrical hollow hole, theinner diameter of the valve head portion hollow hole being decreasedwith increasing height and being equal to the inner diameter of a hollowhole of the hollow shaft portion at the upper end of the barrel section.Thus, it has become possible to carry out cold forging of the valve headportion using a particularly highly heat resistant material whoseforming by conventional cold forging has been considered impossible.

That is, as mentioned earlier, no force is exerted on (the die is notapplied to) the middle to lower part of the diameter-increased section(maximum outer diameter part and the surrounding part) of the valve headportion semifinished product. Instead, the parts upward thereof arenecked down meticulously, little by little, a plurality of times. Thishas resulted in a success in the forming, by cold forging, of the valvehead portion from a highly heat resistant material which has so far beenregarded as impossible to perform.

The number of the necking processes differs according to the size orshape of the hollow engine valve. Provided that the number of thenecking processes is N (N is a positive integer, and N≧2), a number ofthe order of 8 to 15 is considered to be the appropriate number of theprocesses for the hollow engine valve in which the diameter of thehollow shaft portion is of the order of 6 mm, the inner diameter of thehollow hole in the hollow shaft portion is of the order of 3 mm, theouter diameter of the diameter-increased section of the valve headportion is of the order of 30 mm, and the maximum inner diameter of thehollow hole in the diameter-increased section of the valve head portionis of the order of 10 mm (the standard size of a hollow engine valve foruse in the engine of an ordinary-sized passenger car). With the valvehead portion of the hollow engine value of this size, if the number ofthe necking processes is less than 8, each necking process imposes anunbearable burden, causing buckling or cracking, or causing failure inthe smooth performance of the necking process. It is not that the largerthe number of the processes, the better results are obtained. As thenumber of the processes increases, preparations have to be made for moreof the dies, the scale of equipment becomes greater, working islaborious, and the working time lengthens. For 15 or more of theprocesses, the finishing accuracy and the smoothness of the process arenot very different. Thus, the upper limit is considered to be a numberof the order of 15.

As discussed above, no force is exerted on the middle to lower part ofthe diameter-increased section (maximum outer diameter part and thesurrounding part) of the valve head portion, and the number of thenecking processes is set at a number in the appropriate range, wherebysuccess is achieved in the forming, by cold forging, of the valve headportion from the particularly highly heat resistant material. As aresult, devices or apparatuses necessary for hot forging have becomeunnecessary, and the production line has been downscaled and simplifiedsuccessfully. Moreover, time for heating the members, and time forcooling them can be eliminated, thus shortening the manufacturing timeand contributing to cost reduction. Furthermore, cold forging is highlyaccurate compared with hot forging, so that the remarkable effect ofachieving an improvement in the quality of the product could beexhibited.

According to the invention of Solution 1 or Solution 2 as the presentinvention, the first step for obtaining the valve head portionsemifinished product is described in detail. Thus, the present inventioncan be practiced concretely. That is, the invention of Solution 1 orSolution 2 uses as the material a cylindrical solid round bar comprisinga material having heat resistance comparable to or better than the heatresistance of the material for the hollow shaft portion, the solid roundbar having a diameter larger than the minimum outer diameter of thevalve head portion as the finished product, but smaller than the maximumouter diameter of the valve head portion as the finished product. Such amaterial can be obtained easily by forming a round bar to apredetermined diameter, followed by cutting. Forming of the hollow holecan be performed easily by punching or forging (hot, warm or cold), andforming of the diameter-increased section can be performed easily byforging (hot, warm or cold). It goes without saying that working is moredifficult by cold forging than by hot forging, but cold forging issufficiently available, because it suffices to obtain the semifinishedproduct of by far a simpler shape than the valve head portion finishedproduct.

With the present invention, therefore, in the relatively technicallyeasy first step, namely, in the step of obtaining the semifinishedproduct of the valve head portion, the following two elements arepreformed:

-   -   1. the hollow hole having the inner diameter equal to the        maximum inner diameter of the hollow hole of the valve head        portion as the finished product, and    -   2. the diameter-increased section having the outer diameter        equal to the maximum outer diameter of the diameter-increased        section of the valve head portion as the finished product.        In the second step involving cold forging, which has technically        complicated factors, the above sites 1 and 2 are kept out of        touch, energy is concentrated on a very simple “narrowing” (or        necking) process, and this process is repeated a required number        of times to proceed with necking gradually.

The above measures have brought success in the cold forging of the valvehead portion from an excellently heat resistant material which has thusfar been regarded as an impossible procedure.

To these facts do the essential points of the present invention boildown.

That is, as discussed earlier, the conventional procedure has notescaped from the idea of forming the valve head portion from theparticularly highly heat resistant material by working at a stroke.Thus, such forming has been considered impossible with cold forging.Consequently, various advantages of cold forging have been fullyrecognized, but even a person who attempts the forming has not appeared.

On the other hand, the inventor of the present invention has many years'experience in the development of forging technologies, particularly, inthe field of cold forging, as stated earlier. From this experience, theinventor has hit on the idea that the cold forging of the valve headportion from the particularly highly heat resistant material cannot beperformed at a stroke, but may be achievable if it is divided into theaforementioned first step and second step. Practice based on this ideahas led to success in the forming of the valve head portion with highworking accuracy by cold forging, without defects such as buckles orcracks. As noted here, various effects possessed by cold forging havebeen obtainable in the forming of the valve head portion by working thematerial with particularly satisfactory heat resistance.

According to the invention of Solution 3 or Solution 4 as the presentinvention, the respective processes of the second step in the inventionof Solution 1 or Solution 2 are described in detail. Thus, the secondstep with these inventive aspects can be carried out concretely.Moreover, there is a disclosure that a plurality of the valve headportion semifinished products are transfer-moved, so that the secondstep can be constructed rationally, with waste being avoided. That is,the cold forging step as the second step can be performed, with minimaltime required for working and minimal scale of working equipment.Furthermore, according to the invention of Solution 5 or Solution 6 asthe present invention, there can be obtained a hollow engine valve as afinished product having the valve head portion obtained by the inventionof Solution 1 or Solution 2 or Solution 3 or Solution 4 as the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is an explanation drawing for explaining a first step in amanufacturing method in Embodiment 1 of the present invention.

FIG. 1( b) is an explanation drawing for explaining the first step inthe manufacturing method in Embodiment 1 of the present invention.

FIG. 2( a) is an explanation drawing for explaining a first method ofthe first step in the manufacturing method in Embodiment 1 of thepresent invention.

FIG. 2( b) is an explanation drawing for explaining the first method ofthe first step in the manufacturing method in Embodiment 1 of thepresent invention.

FIG. 2( c) is an explanation drawing for explaining the first method ofthe first step in the manufacturing method in Embodiment 1 of thepresent invention.

FIG. 3( a) is an explanation drawing for explaining a second method ofthe first step in the manufacturing method in Embodiment 1 of thepresent invention.

FIG. 3( b) is an explanation drawing for explaining the second method ofthe first step in the manufacturing method in Embodiment 1 of thepresent invention.

FIG. 3( c) is an explanation drawing for explaining the second method ofthe first step in the manufacturing method in Embodiment 1 of thepresent invention.

FIG. 4 is an explanation drawing for explaining a second step in themanufacturing method in Embodiment 1 of the present invention.

FIG. 5( a) is an explanation drawing for explaining the configuration ofa hollow engine valve in Embodiment 1 of the present invention.

FIG. 5( b) is an explanation drawing for explaining the configuration ofthe hollow engine valve in Embodiment 1 of the present invention.

FIG. 6 is graphs showing the main components of various materials foruse in a valve head portion, a hollow shaft portion, and a shaft endsealing material in Embodiment 1 of the present invention.

FIG. 7( a) is an explanation drawing for explaining the configuration ofa hollow engine valve in Embodiment 2 of the present invention.

FIG. 7( b) is an explanation drawing for explaining the configuration ofthe hollow engine valve in Embodiment 2 of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Modes for carrying out the present invention will be described in detailbelow by reference to the accompanying drawings.

Embodiment 1

As Embodiment 1 of the present invention, a method for production of avalve head portion 1, and a hollow engine valve V having the valve headportion 1 will be described in detail below. The hollow engine valve Vis composed of the valve head portion 1, a hollow shaft portion 2, and ashaft end sealing material 3, as shown in FIG. 5 a and FIG. 5 b. Thatis, the hollow engine valve V is in a configuration in which the valvehead portion 1 is welded to one end of the hollow shaft portion 2, theshaft end sealing material 3 is welded to the other end of the hollowshaft portion 2, and a hollow hole S is provided inside. Sodium (notshown) is enclosed in the hollow hole S. The sodium is enclosed thereinbefore the shaft end sealing material 3 is welded. As the hollow shaftportion 2, there can be used an electric resistance welded pipe formedby rolling up a sheet plate and welding its ends together, or a seamlesspipe without seams. There is no limitation on a welding method forwelding the valve head portion 1 or the shaft end sealing material 3 tothe hollow shaft portion 2, and friction welding, for example, can beused. S1 denotes a hollow hole within the valve head portion 1, and S2denotes a hollow hole within the hollow shaft portion. When the hollowengine valve V is used for an intake valve, there is no need to enclosesodium within the hollow hole S, and the hollow hole S may remainhollow.

In connection with the materials for the valve head portion 1, thehollow shaft portion 2, and the shaft end sealing material 3, they arearranged based on heat resistance as follows:

Materials having particularly high resistance to heat (material group A). . . Used for the valve head portion 1.

Materials having the next highest resistance to heat (material group B). . . Used for the hollow shaft portion 2.

Materials having ordinary resistance to heat (material group C) . . .Used for the shaft end sealing material 3.

However, the above arrangement is predicated on the use of the hollowengine valve V for an exhaust valve. If the hollow engine valve V isused for an intake valve, there is no harm in constituting all of thevalve head portion 1, the hollow shaft portion 2, and the shaft endsealing material 3 from the above material group C (materials havingordinary heat resistance).

An example of the above material group A will be named concretely below.For the main components of each material, reference is requested to FIG.6.

NCF47W (nickel-based steel)

SUH35 (austenitic manganese-based steel)

Inconel 751 (nickel-based steel)

“Inconel” is a registered trademark of Special Metals Corporation(formerly, International Nickel Company), and the Inconel products areavailable in various kinds depending on the proportions of elements,such as chromium, niobium and molybdenum, added to nickel as the base.All these products have particularly good heat resistance, but theirworking is difficult.

Next, an example of the above material group B will be named concretelybelow. For the main components of each material, reference is requestedto FIG. 6.

SUS304 (austenitic nickel-based steel)

SUS430 (ferritic stainless steel)

SUH11 (martensitic stainless steel)

SUS304 is a representative stainless steel sometimes called 18-8. It isexcellent in workability, corrosion resistance, and heat resistance, buthardens greatly upon working, and is thus considered to be slightlyunsuitable for cold forging. SUS430 is a representative steel amongchromium stainless steels called 18Cr. It is less expensive than the18-8 products, but is slightly inferior in workability and corrosionresistance. For SUH11, reference is requested to the followingparagraph.

Finally, an example of the above material group C will be namedconcretely below. For the main components, reference is requested toFIG. 6.

SUH11 (martensitic stainless steel)

SUH11 is a martensitic heat resistant steel, and is chromium-based. Itsheat resistance is inferior to that of SUS304 and SUS430, but itsworkability is satisfactory. Since this material has a certain degree ofheat resistance, it can be used for the hollow shaft portion 2, asmentioned above.

The materials for the valve head portion 1, the hollow shaft portion 2,and the shaft end sealing material 3 are as described above. When thehollow engine valve V is used for an exhaust valve, the material for thevalve head portion 1 is selected from the material group A, the materialfor the hollow shaft portion 2 is selected from the material group B,and the material for the shaft end sealing material 3 is selected fromthe material group C. Needless to say, however, it poses no problem evenif the material for the hollow shaft portion 2 is selected from thematerial group A, and the material for the shaft end sealing material 3is selected from the material groups A and B. When the hollow enginevalve V is used for an intake valve, the materials for the valve headportion 1, the hollow shaft portion 2, and the shaft end sealingmaterial 3 can all be selected from the material group B or group C.Even if these materials are selected from the group A, there would, ofcourse, be technically no problem, but this selection is meaninglessfrom the aspect of working or price.

The formation of the valve head portion, as the core of Embodiment 1 ofthe present invention, will be describe in detail below.

<First Step>

FIGS. 1 a, 1 b show the first step of Embodiment 1 of the presentinvention. FIG. 1 a shows a semifinished product 11 of the valve headportion 1 in a longitudinal sectional view. The semifinished product 11has a disk-shaped diameter-increased section 111 and a cylindricalbarrel section 112 formed integrally. The lower end of the barrelsection 112 is continuously connected to the upper end of thediameter-increased section 111, and the junction between them is ingentle curves, as shown in FIG. 1 a. A hollow hole S11 of a cylindricalshape having a bottomed lower end is formed inside the semifinishedproduct 11, and the upper end of the hollow hole S11 opens at the uppersurface of the barrel section 112, while the lower end of the hollowhole S11 is bottomed in the diameter-increased section 111. An upperpart of the diameter-increased section 111 and the whole of the barrelsection 112 of the semifinished product 11 are narrowed (necked) by coldforging to obtain a finished product of the valve head portion 1 asshown in FIG. 1 b. In FIG. 1 b, 1 a denotes a diameter-increasedsection, and 1 b denotes a barrel section. In the valve head portion 1as the finished product, it is difficult to determine the boundarybetween the diameter-increased section 1 a and the barrel section 1 b.In FIG. 1 b, however, the diameter-increased section 1 a and the barrelsection 1 b are separated at the site where the curvature of the curveof the contour of the sectional view sharply increases. S1 denotes ahollow hole of a cylindrical shape having a bottomed lower end, and theupper end of the hollow hole S1 opens at the upper surface of the barrelsection 1 b, while the lower end of the hollow hole S1 is bottomedinside the diameter-increased section 1 a.

In FIG. 1 a, h11 denotes the entire height of the semifinished product11, h12 denotes the height of the diameter-increased section 111, h13denotes the height of the barrel section 112, h14 denotes the height(depth) of the hollow hole S11, Φ10 denotes the outer diameter of thebarrel section 112, Φ12 denotes the maximum outer diameter of thediameter-increased section 111, and Φ11 denotes the inner diameter ofthe hollow hole S1. In FIG. 1 b, h15 denotes the entire height of thevalve head portion 1 as the finished product, h16 denotes the height ofthe diameter-increased section 1 a, h17 denotes the height of the barrelsection 1 b, h18 denotes the height (depth) of the hollow hole S1, Φ14denotes the outer diameter of the upper end part of the barrel section 1b, Φ12 denotes the maximum outer diameter of the diameter-increasedsection 111, Φ11 denotes the maximum inner diameter of the hollow holeS1, and Φ13 denotes the inner diameter of the upper end part of thehollow hole S1.

Here, the entire height h15 of the valve head portion 1 as the finishedproduct is larger than the entire height h11 of the semifinished product11 (h11<h15); the height (depth) h18 of the hollow hole S1 is largerthan the height (depth) h14 of the hollow hole S11 (h14<h18); the heighth12 of the diameter-increased section 111 is nearly equal to the heighth16 of the diameter-increased section 1 a (h12≈h16); the height h17 ofthe barrel section 1 b is larger than the height h13 of the barrelsection 112 (h13<h17); the maximum outer diameter of thediameter-increased section 111 is equal to the maximum outer diameter ofthe diameter-increased section 1 a (Φ12); the outer diameter Φ10 of theupper end part of the barrel section 112 is larger than the outerdiameter Φ14 of the upper end part of the barrel section 1 b (Φ14<Φ10);the inner diameter of the hollow hole S11 is equal to the maximum innerdiameter of the hollow hole S1 (Φ11); and the inner diameter Φ11 of thehollow hole S11 is larger than the inner diameter Φ13 of the upper endpart of the hollow hole S1 (Φ13<Φ11).

FIGS. 2 a to 2 c show the first method for obtaining the semifinishedproduct 11. As shown in FIG. 2 a, preparations are made for a solidround bar 2A comprising a material selected from the material group A(see FIG. 6). In Embodiment 1, NCF47W was used (for the main components,see FIG. 6). The outer diameter of the solid round bar 2A is Φ10, thesame as the outer diameter of the barrel section 112 of the semifinishedproduct 11, and the height h20 of the solid round bar 2A is smaller thanthe height h11 of the semifinished product 11 (h20<h11).

A hollow hole 2C is formed in the upper surface of the solid round bar2A by means of a punch to obtain a cup-shaped intermediate member 2B(FIG. 2 b). The hollow hole 2C has a height (depth) h22 which is nearlya half of the entire height h21 of the intermediate member 2B. On thisoccasion, the outer diameter of the intermediate member 2B is renderedthe same as the outer diameter Φ10 of the solid round bar 2A, with theresult that the height h21 of the intermediate member 2B is larger thanthe height h20 of the solid round bar 2A (h20<h21). The inner diameterof the hollow hole 2C is equal to the inner diameter Φ11 of the hollowhole S11 of the semifinished product 11 (FIG. 2 c).

Then, a lower part of the intermediate member 2B is shaped by forging toforma diameter-increased section 111. On this occasion, the type offorging does not matter. That is, any of cold forging, warm forging, andhot forging may be used. Since this step is an intermediate step,accuracy as required in the second step to be described later is notdemanded. However, the following three points are of importance: theouter diameter of the upper part of the intermediate member 2B ismaintained at the outer diameter Φ10 of the barrel section of thesemifinished product 11; the inner diameter of the hollow hole 2C ismaintained at the inner diameter Φ11 of the hollow hole S11 of thesemifinished product 11; and when the lower part of the intermediatemember 2B is converted into the diameter-increased section 111, itsmaximum outer diameter is rendered the maximum outer diameter Φ12 of thediameter-increased section 111 of the semifinished product 11. Duringthis process, the hollow hole 2C (height h22) is slightly deepened toform the hollow hole S11 having a height (depth) h14. In this manner,the semifinished product 11 (FIG. 2 c) is obtained from the solid roundbar 2A (FIG. 2 a) via the intermediate member 2B (FIG. 2 b).

FIGS. 3 a to 3 c show the second method for obtaining the semifinishedproduct 11. As shown in FIG. 3 a, preparations are made for a solidround bar 3A comprising a material selected from the material group A(see FIG. 6). In Embodiment 1, NCF47W was used. The outer diameter ofthe solid round bar 3A is Φ10, the same as the outer diameter of thebarrel section 112 of the semifinished product 11, and the height h30 ofthe solid round bar 3A is smaller than the height h11 of thesemifinished product 11 (h30<h11). The height h30 is equal to the heighth20 of the aforementioned solid round bar 2A (h30=h20).

A lower part of the solid round bar 3A is shaped by forging to form ahat-shaped solid intermediate member 3B having a diameter-increasedsection 3C (FIG. 3 b). On this occasion, the type of forging does notmatter. That is, any of cold forging, warm forging, and hot forging maybe used. Since this step is an intermediate step, accuracy as requiredin the second step to be described later is not demanded. However, thefollowing two points are of importance: the outer diameter of the upperpart of the intermediate member 3B is maintained at the outer diameterΦ10 of the barrel section of the semifinished product 11; and when thelower part of the intermediate member 3B is converted into thediameter-increased section 3C, the maximum outer diameter of thediameter-increased section 3C is rendered the maximum outer diameter Φ12of the diameter-increased section 111 of the semifinished product 11.During this process, the height h31 of the intermediate member 3B isslightly decreased. That is, h31<h30.

Then, a hollow hole S11 having a height (depth) h14 and an innerdiameter of Φ11 is formed in an upper surface of the intermediate member3B by means of a punch. During this process, an upper part of theintermediate member 3B is extended to form a barrel section 112 with aheight of h13 (FIG. 3 c). In this manner, the semifinished product 11(FIG. 3 c) is obtained from the solid round bar 3A (FIG. 3 a) via theintermediate member 3B (FIG. 3 b). On this occasion, the following twopoints are of importance: the outer diameter of the barrel section 112is maintained at Φ10; and the maximum outer diameter of thediameter-increased section 111 is maintained at Φ12.

<Second Step>

Next, FIG. 4 illustrates details of the process for cold forging in thesecond step. In FIG. 4, D1, D2, D(m−1), Dm, D(n−1), and Dn denote dies.Here, Dn represents the n-th (last) die, Dm represents the m-th die,m<n, and m and n each denote a positive integer of 3 or larger. D(m−1)represents the die directly before the die Dm, and D(n−1) represents thedie directly before the die Dn. The dies D1, D2, D(m−1), Dm, D(n−1), andDn have central forming holes M1, M2, M(m−1), Mm, M(n−1), and Mn whoseinner diameters decrease sequentially. RM represents a ram to which thedies D1, D2, D (m−1), Dm, D (n−1), and Dn are fixed.

The numeral 11 denotes a semifinished product, the numerals 11 a, 11 mand 11 n denote intermediate works, and the numeral 1 denotes a valvehead portion as a finished product. Moreover, PB represents a press bedwhere the semifinished product 11, the intermediate works 11 a, 11 m and11 n, and the finished product of the valve head portion 1 are fixed. Toachieve the appropriate state of fixing of the dies D1, D2, D (m−1), Dm,D (n−1), and Dn to the ram RM, and the appropriate state of fixing ofthe semifinished product 11, the intermediate works 11 a, 11 m and 11 n,and the finished product of the valve head portion 1 to the press bedPB, various fixtures are needed without doubt. However, their indicationwill make the accompanying drawings complicated and difficult tounderstand. Thus, illustrations of the various fixtures and so on areall omitted. Furthermore, apparatuses concerned with transfer movement(to be described later) for use in this step are all omitted from theillustrations herein.

Next, the actions of the second step will be described. In FIG. 4, thedie D1 is located above the semifinished product 11, the die D2 islocated above the intermediate work 11 a, the die Dm is located abovethe intermediate work 11 m, and the die Dn is located above theintermediate work 11 n. In the initial state, it is only natural thatthe semifinished product 11 is located below the die D1, and none of theintermediate works 11 a, 11 m, 11 n and the finished product of thevalve head portion 1 are present. In this state, the ram RM is lowered(in the y direction).

Once the ram RM lowers, the forming hole M1 of the die D1 necks down theupper part of the diameter-increased section 111 and the barrel section112 in the semifinished product 11 to form the semifinished product 11into the intermediate work 11 a. The inner diameter of the forming holeM1 is slightly smaller than the outer diameter Φ10 of the barrel section112 of the semifinished product 11. Thus, the outer diameter Φ10 of thebarrel section 112 of the semifinished product 11 is slightly decreasedto obtain the intermediate work 11 a, whose height is somewhat largerthan the height h11 of the semifinished product 11 (see FIG. 1 a). Thehollow hole S11 of the semifinished product 11 is also narrowed tobecome the hollow hole S11 a of the intermediate work 11 a whose innerdiameter Φ11 has been slightly decreased. However, the die D1 does notpress the region from the middle part to the lower part of thediameter-increased section 111 (the part with the maximum outer diameterand the surrounding part) of the semifinished product 11. Thus, thelower end part of the hollow hole S11 is not pressed, and the innerdiameter at this site is maintained at Φ11. Hence, needless to say, themaximum outer diameter of the diameter-increased section 111 a of theintermediate work 11 a is also maintained at Φ12.

Upon completion of the first necking, the ram RM is raised, and thesemifinished product 11 has turned into the intermediate work 11 a. Onthis occasion, by the action of the fixture (not shown), theintermediate work 11 a is fixed to the press bed PB, and thus does notascend together with the die D1. When the ascent of the ram RM ends, theintermediate work 11 a (formerly, the semifinished product 11) isreleased from fixing, and transfer-moved (in the x direction) rightwardto the next position in FIG. 5 by a moving device (not shown),whereafter it is located directly below the next die D2 and fixed again.The transfer movement refers to such a movement as to displace the worksequentially, by one position at a time. When the intermediate work 11 a(former semifinished product 11) is transfer-moved rightward, a newsemifinished product 11 is fixed to the vacant position.

At this stage, therefore, the semifinished product 11 and theintermediate work 11 a are fixed onto the press bed PB. In this state,the ram RM is lowered (in the y direction) to press-form thesemifinished product 11 by the die D1 and the intermediate work 11 a bythe die D2. Upon completion of the press forming, the semifinishedproduct 11 and the intermediate work 11 a are transfer-moved rightward,and a new semifinished product 11 is fixed to the left end of the pressbed PB.

The m-th intermediate work 11 m and the m-th die Dm are shown in thecenter of FIG. 4. Rightwardly of them, the last intermediate work 11 nand the last die Dn are shown. The last intermediate work 11 n ispress-formed by the die Dn, whereby it is converted into the valve headportion 1 as the finished product, and is then transfer-moved rightward.The transfer-moved valve head portion 1 as the finished product is shownat the right end of FIG. 4. The finished product, valve head portion 1,is removed from the press bed PB.

In the foregoing manner, each time the ram RM is lowered (in the ydirection), the semifinished product 11, and the intermediate works 11a, 11 m, 11 n are transfer-moved rightward (in the x direction). Also,the valve head portion 1 as the finished product is removed from thepress bed PB, and a new semifinished product 11 is fixed to the left endof the press bed PB. In this manner, forming by cold forging is carriedout continuously, whereby each descent of the ram RM results in theproduction of one valve head portion 1 as the finished product. Thus,this procedure is very rational and efficient. Generally, the number ofthe forming processes being large is pointed out as a drawback of coldforging, but this drawback is dissolved by adopting the above-describedtransfer movement.

As shown in FIG. 4, the dies D1, D2, Dm and Dn all press-form the upperparts of the diameter-increased sections 111, 111 a, 111 m, 111 n andbarrel sections 112, 112 a, 112 m, 112 n of the semifinished product 11and the intermediate works 11 a, 11 m, 11 n, and do not touch the centerto the lower part of the diameter-increased sections 111, 111 a, 111 m,111 n (the part of the maximum outer diameter and the surrounding part).Thus, the maximum outer diameter of the diameter-increased sections 111,111 a, 111 m, 111 n is maintained unchanged at Φ12, which becomes themaximum outer diameter Φ12 of the diameter-increased section 1 a of thevalve head portion 1 as the finished product.

The parts other than the lower end parts of the hollow holes S11, S11 a,S11 m, S11 n of the semifinished product 11 and the intermediate works11 a, 11 m, 11 n have inner diameters decreased as a result of pressforming. On the other hand, the lower end parts of the hollow holes S11,S11 a, S11 m, S11 n are not subjected to press forming, so that theirmaximum inner diameter is kept at Φ11, which leads to the maximum innerdiameter Φ11 of the hollow hole of the valve head portion 1 as thefinished product.

N, which is the number of the steps or processes in the second step, isbasically a natural number of 2 or greater, but concretely, is differentdepending on the size or shape of the hollow engine valve demanded.Currently, for the hollow engine valve in which the diameter of thehollow shaft portion is of the order of 6 mm, the inner diameter of thehollow hole in the hollow shaft portion is of the order of 3 mm, theouter diameter of the diameter-increased section of the valve headportion is of the order of 30 mm, and the maximum inner diameter of thehollow hole in the diameter-increased section of the valve head portionis of the order of 10 mm (the standard size of a hollow engine valve foruse as the engine of an ordinary-sized passenger car), the appropriatenumber of the processes is considered to be of the order of 8 to 15. Thereason for this numerical restriction is as described earlier. InEmbodiment 1, cold forging of the hollow engine valve of the above sizewas attempted in various numbers, and the optimum number of theprocesses was found to be in the neighborhood of 12.

In connection with transfer movement, there may be a method in which theintermediate work 11 a (the former semifinished product 11) is merelytransfer-moved rightward during the first transfer movement, and a newsemifinished product 11 is not placed directly below the die D1. Withthis method, a new semifinished product 11 is placed directly below thedie D1 at the time of the odd-numbered transfer movement, like the thirdtransfer movement, the fifth transfer movement, and so on. By so doing,the works on the press bed PB are placed on every other position, withthe result that the resistance during pressing is halved. Time forobtaining the same number of the products is twice that taken inordinary transfer movement, but many advantages are brought, such as alabor saving for equipment and an increase in durability of theequipment because of a reduction in the pressure of the press.

In the above configuration, the works and the dies were both arrangedlinearly, but various applied forms are naturally conceivable, such ascircumferential arrangement or zigzag arrangement. Moreover, the ram RMwas placed above, and the press bed PB was placed below. However, theram RM may be placed below, while the press bed PB may be placed above,and the ram RM may be raised at the time of forming.

The second step described above is performed by cold forging inEmbodiment 1, but needless to say, can be carried out by warm forging.Cold forging refers normally to forging at the ordinary temperature, butas far as the temperature range is concerned, forging at a temperatureof the order of 0° C. to 250° C. may be called cold forging. The rangeof the temperature for warm forging is normally regarded as 600° C. to850° C. It follows that forging at 250° C. to 600° C. is cold forging ifthe temperature is lower, and warm forging if the temperature is higher.Forging at a temperature in excess of 850° C. is deemed to be hotforging.

The details of the second step have been described above. As a result ofthe second step, the forming of the valve head portion 1 is completed.Then, the three members shown in FIG. 5 a, i.e., the valve head portion1, the hollow shaft portion 2, and the shaft end sealing material 3, areall integrated as in FIG. 5 b, that is, by securing the valve headportion 1 to one end of the hollow shaft portion 2, and the shaft endsealing material 3 to the other end of the hollow shaft portion 2, bywelding. By so doing, the hollow engine valve V is completed. InEmbodiment 1, friction welding was used as the welding method. When thehollow engine valve V is used for an exhaust valve, sodium is enclosedin the hollow hole S before the shaft end sealing material 3 is welded,although this not shown in FIG. 5 b.

As shown in FIG. 5 b, the position of welding between the valve headportion 1 and the hollow shaft portion 2 is a high position rather closeto the middle of the hollow engine valve V. This is one of the merits ofthe method of the present invention which forms the valve head portion 1by cold forging. When the weld line can be separated from the leadingend of the valve head portion 1 this far, the weld line is alwayslocated outside the cylinder during engine operation. Thus, theinfluence of heat is so slight that the problem of concern over strengthdue to welding is completely resolved. Even if a material inferior inheat resistance to the material of the valve head portion 1 is used asthe material for the hollow shaft portion 2, no problem arises.

EMBODIMENT 2

A hollow engine valve W in Embodiment 2 of the present invention iscomposed of a valve head portion 1 and a hollow shaft portion 20, asshown in FIG. 7 a and FIG. 7 b. That is, the hollow engine valve W is ina configuration in which the hollow shaft portion 20 is sealed at oneend, the valve head portion 1 is welded to the other end of the hollowshaft portion 20, and a hollow hole SS is provided inside. Sodium (notshown) is enclosed in the hollow hole SS. The sodium is enclosed thereinbefore the valve head portion 1 and the hollow shaft portion 20 arewelded. As the hollow shaft portion 20, there can be used an electricresistance welded pipe formed by rolling up a sheet plate and weldingits ends together, or a seamless pipe without seams and sealed at oneend, or a solid round bar having a hollow hole S 20 bored by a drill.There is no limitation on a welding method for welding the valve headportion 1 to the hollow shaft portion 2, and friction welding, forexample, can be used. When the hollow engine valve W is used for anintake valve, there is no need to enclose sodium within the hollow holeSS, and the hollow hole SS may remain hollow. The method of producingthe valve head portion 1 is the same as in Embodiment 1 for both of thefirst step and the second step, and thus its detailed description isomitted.

As the material for the valve head portion 1 of Embodiment 2, thematerial group A mentioned in the paragraphs of Embodiment 1 can beused. As the material for the hollow shaft portion 20 of Embodiment 2,the material group B mentioned in the paragraphs of Embodiment 1 can beused.

INDUSTRIAL APPLICABILITY

As described above, the present invention discloses the contents of thetechnology which can perform the formation of the valve head portion inthe hollow engine valve, using the material having particularly highheat resistance, by cold forging. This is the technology whichtechnicians in the art have regarded, so to speak, as “the technology ofdreams”, that is, the one which they have considered to be ideal, butthey have judged to be unachievable at the present time. Accordingly,this technology rendered feasible means to provide a so-calledrevolutionary method in the method of producing a hollow engine valve.

As stated earlier, the material used in the valve head portion of thehollow engine valve has particularly high hardness. Thus, poor accuracydue to hot forging and heavy working labor involving a waiting timeuntil cooling have to be endured, and a low economic efficiency due tothe need for soonest replacement of a gun drill having cemented carbideblades has to be tolerated, in forming the valve head portion. The factthat cold forging of the valve head portion has become possible by thepresent invention, therefore, has aroused the possibility forrefurbishing the line in the plant for the hollow engine valve. That is,a shift has been realized toward a novel manufacturing system which canmass-produce high accuracy hollow engine valves rationally andinexpensively by simple equipment.

The engine valve essentially accompanies an engine, and is used not onlyin engines for vehicles such as passenger cars and trucks, but also inengines for airplanes and engines for ships. Particularly for an exhaustvalve, a hollow engine valve having sodium enclosed therein is oftenused. Thus, the method for production of the hollow engine valve of thepresent invention can be one which provides an epoch-making technicalinnovation in the engine manufacturing process.

In an intake valve as well as in an exhaust valve, the adoption of ahollow engine valve leads to the weight reduction of the entire engine,and also results in cost reduction attributed to the rationalization ofthe materials. Thus, a future trend seems to be toward the hollowing ofthe engine valve in an intake valve as well. In such a stream, thepresent invention is expected to have great industrial applicability inindustries concerned with the production of engines.

DESCRIPTION OF THE NUMERALS

-   -   1 Valve head portion    -   1 a Diameter-increased section    -   1 b Barrel section    -   11 Semifinished product    -   11 a Intermediate work    -   11 m Intermediate work    -   11 n Intermediate work    -   111 Diameter-increased section    -   111 a Diameter-increased section    -   111 m Diameter-increased section    -   111 n Diameter-increased section    -   112 Barrel section    -   112 a Barrel section    -   112 m Barrel section    -   112 n Barrel section    -   2 Hollow shaft portion    -   20 Hollow shaft portion    -   2A Solid round bar    -   2B Intermediate member    -   2 c Hollow hole    -   3 Shaft end sealing material    -   3A Solid round bar    -   3B Intermediate member    -   3C Diameter-increased section    -   D1 Die    -   D2 Die    -   D(m−1) Die    -   Dm Die    -   D(n−1) Die    -   M1 Forming hole    -   M2 Forming hole    -   M(m−1) Forming hole    -   Mm Forming hole    -   M(n−1) Forming hole    -   PB Press bed    -   RM Ram    -   S Hollow hole    -   SS Hollow hole    -   S1 Hollow hole    -   S11 Hollow hole    -   S11 a Hollow hole    -   S11 m Hollow hole    -   S11 n Hollow hole    -   S2 Hollow hole    -   S20 Hollow hole    -   V Hollow engine valve    -   W Hollow engine valve    -   h11 Height    -   h12 Height    -   h13 Height    -   h14 Height    -   h15 Height    -   h16 Height    -   h17 Height    -   h18 Height    -   h20 Height    -   h21 Height    -   h22 Height    -   h30 Height    -   h31 Height    -   Φ10 Outer diameter    -   Φ11 Inner diameter    -   Φ12 Maximum outer diameter    -   Φ13 Inner diameter    -   Φ14 Outer diameter

The invention claimed is:
 1. A method for production of a valve head portion of a hollow engine valve, in which the hollow engine valve has a hollow shaft portion, and the valve head portion comprising a material having heat resistance comparable to or better than heat resistance of a material for the hollow shaft portion, the valve head portion has a valve head portion hollow hole opening on a side welded to the hollow shaft portion, the valve head portion hollow hole is formed to have an increased diameter in a diameter-increased section of the valve head portion, and a maximum inner diameter of the valve head portion hollow hole is larger than a maximum outer diameter of the hollow shaft portion, the method comprising: a first step of producing a valve head portion semifinished product by working the material having the heat resistance comparable to or better than the heat resistance of the material for the hollow shaft portion to form the valve head portion semifinished product, in which the valve head portion semifinished product has a cylindrical barrel section and a diameter-increased section at one end of the barrel section, the diameter-increased section being integral with the barrel section, when the diameter-increased section is located below, a maximum outer diameter of the diameter-increased section is equal to a maximum outer diameter of the diameter-increased section of the valve head portion as a finished product, and the valve head portion semifinished product has a cylindrical hollow hole having an inner diameter equal to the maximum inner diameter of the valve head portion hollow hole of the finished product, the cylindrical hollow hole opening at an upper end, and having a lower end bottomed in the diameter-increased section, the first step including: using a cylindrical solid round bar as a material, the solid round bar comprising the material having the heat resistance comparable to or better than the heat resistance of the material for the hollow shaft portion, the solid round bar having a diameter larger than a minimum outer diameter of the valve head portion as the finished product, but smaller than a maximum outer diameter of the valve head portion as the finished product; while rendering a circular surface at one end of the solid round bar an upper surface, and a circular surface at another end of the solid round bar a lower surface, forming a cylindrical hollow hole opening at an upper end, and being bottomed at a lower end, an inner diameter of the cylindrical hollow hole being equal to the maximum inner diameter of the valve head portion hollow hole of the valve head portion as the finished product; and then increasing a diameter of a lower part of the solid round bar by forging to bring a maximum outer diameter of the lower part into conformity with the maximum outer diameter of the diameter-increased section of the valve head portion as the finished product, thereby producing the valve head portion semifinished product having the diameter-increased section whose maximum outer diameter coincides with the maximum outer diameter of the diameter-increased section of the valve head portion as the finished product, the valve head portion semifinished product having the cylindrical hollow hole having the inner diameter equal to the maximum inner diameter of the valve head portion hollow hole of the valve head portion as the finished product, the cylindrical hollow hole having the lower end bottomed; and a second step of gradually necking down an upper part of the diameter-increased section and the barrel section of the valve head portion semifinished product by cold forging in a plurality of stages, that is, gradually necking down the upper part of the diameter-increased section and the barrel section by use of dies for pressing the upper part of the diameter-increased section and the barrel section, inner diameters of the dies being decreased little by little as the stage advances, a number of the dies used being equal to a number of processes for necking, thereby obtaining the valve head portion as the finished product configured such that the maximum inner diameter of the valve head portion hollow hole in the diameter-increased section is maintained at the inner diameter of the cylindrical hollow hole, the inner diameter of the valve head portion hollow hole being decreased with increasing height and being equal to an inner diameter of a hollow hole of the hollow shaft portion at an upper end of the barrel section.
 2. The method for production of a valve head portion of a hollow engine valve according to claim 1, further comprising, in the second step: installing a ram where the dies are fixed, and a press bed where works are fixed, at spaced locations with the ram being located above and the press bed being located below; fixing N of the dies, which corresponds to a number of the processes (N of the processes) for cold forging, at equally spaced intervals to the ram in a sequence of the processes for necking, N being a positive integer and N≧2; locating the valve head portion semifinished products on the press bed such that a first of the valve head portion semifinished products lies below a first of the dies; lowering the ram to perform first forging of the first valve head portion semifinished product for necking by the first die; raising the ram, and transfer-moving the first valve head portion semifinished product to be located below a second of the dies, and simultaneously locating a second of the valve head portion semifinished products below the first die; lowering the ram to perform second forging of the first valve head portion semifinished product for necking by the second die, and simultaneously perform first forging of the second valve head portion semifinished product for necking by the first die; raising the ram, and transfer-moving the first valve head portion semifinished product to be located below a third of the dies, and the second valve head portion semifinished product to be located below the second die, and simultaneously locating a third of the valve head portion semifinished products below the first die; lowering the ram N times and transfer-moving a plurality of the valve head portion semifinished products, in such a manner, and when the first valve head portion semifinished product becomes a valve head portion finished product as a first valve head portion finished product, removing the first valve head portion finished product from the press bed; and further continuing the processes to obtain a plurality of the valve head portion finished products.
 3. The method for production of a valve head portion of a hollow engine valve according to claim 1, further comprising, in the second step: installing a ram where the dies are fixed, and a press bed where works are fixed, at spaced locations with the ram being located below and the press bed being located above; fixing N of the dies, which corresponds to a number of the processes (N of the processes) for cold forging, at equally spaced intervals to the ram in a sequence of the processes for necking, N being a positive integer and N≧2; locating the valve head portion semifinished products on the press bed such that a first of the valve head portion semifinished products lies above a first of the dies; raising the ram to perform first forging of the first valve head portion semifinished product for necking by the first die; lowering the ram, and transfer-moving the first valve head portion semifinished product to be located above a second of the dies, and simultaneously locating a second of the valve head portion semifinished products above the first die; raising the ram to perform second forging of the first valve head portion semifinished product for necking by the second die, and simultaneously perform first forging of the second valve head portion semifinished product for necking by the first die; lowering the ram, and transfer-moving the first valve head portion semifinished product to be located above a third of the dies, and the second valve head portion semifinished product to be located above the second die, and simultaneously locating a third of the valve head portion semifinished products above the first die; raising the ram N times and transfer-moving a plurality of the valve head portion semifinished products, in such a manner, and when the first valve head portion semifinished product becomes a valve head portion finished product as a first valve head portion finished product, removing the first valve head portion finished product from the press bed; and further continuing the processes to obtain a plurality of the valve head portion finished products. 